This application is a continuation-in-part of Finney et al., U.S. Ser. No. 231,821, filed Aug. 12, 1988, entitled CONSTRAINED UNIFORM FIELD GEL ELECTROPHORESIS.
This invention concerns apparatus for gel electrophoresis of high molecular weight DNA.
It is known that an electric field can be used to move DNA molecules through semi-solid gels in order to separate the molecules on the basis of size. Small molecules move faster than large ones because they encounter less resistance from the gel matrix. Molecules up to about 30,000 bases in length can be separated in this way; larger molecules (up to many million bases in length) all move at about the same rate, and thus are more difficult to separate.
Recently, a few systems have emerged that exhibit a reduction in the limitations experienced during standard electrophoresis. These systems depend on periodic changes in the direction of the electric field in the gel. For example, Cantor et al. (U.S. Pat. Nos. 4,473,452 and 4,695,548) describe a system in which generally non-uniform electric fields change direction in the plane of the gel. While the mechanism by which this electrophoresis method operates to move large DNA molecules is not entirely understood, Cantor et al. propose that the application of alternating fields causes a coiled DNA molecule to be squeezed into the gel matrix by orientating itself first along the general direction of one of the fields and then along the general direction of the other, and so on. Moreover, they believe that by using gradient fields, rather than uniform fields, a shearing effect is produced that helps stretch the DNA molecule in a desired direction. Carle et al. (U.S. Pat. No. 4,737,251) describe a method for field inversion electrophoresis. In this method, the electric field is changed in direction by 180.degree.. Net migration is achieved by using a longer time or higher voltage in one direction than in the other direction. Chu et al. (234 Science 1582, 1986) describe a method of electrophoresis termed "contour clamped" or "homogenous electric field" electrophoresis. This method of electrophoresis depends on a complex network of diodes and resistors to fix the electric potential at a number of points around the gel, and thus fix the electric field in the gel. This method can also be used for electrophoresis in non-uniform fields. Laas et al. (U.S. Pat. No. 4,740,283) and a 1987 Beckman Instrument advertisement entitled GENELINE.TM. describe a pulse field gradient gel electrophoresis apparatus having electrode arrays oriented to provide three dimensional fields across the face of the gel. Serwer (U.S. Pat. No. 4,693,804) describes an apparatus for conducting electrophoresis in two dimensions without need to move a gel after electrophoresis in a first dimension. Barriers are provided to limit an electric field to a rectangular shape and prevent passage of electricity. After electrophoresis in a first dimension these barriers are removed and new barriers erected to form a new electric field in a rectangular shape oriented at 90.degree. to the first.